Abstract

Distributed roughness with nominal height k in a boundary layer with displacement thickness δ* is known to lead to early non‐Tollmien–Schlichting–Schubauer (TS) transition when k/δ* exceeds unity in nonaccelerated flows. But could commonly occurring milder random roughness enhance the regular phase‐conditioned TS mechanisms and also lead to early transition? The new affirmative answer rests on extensive hot‐wire explorations and smoke‐wire visualization of flow over smooth and rough walls. The effects appear to be threefold. First, the low‐inertia fluid in the valleys between the elements evidently respond more readily to free‐stream disturbances. Second, once the TS fluctuations commence, they grow faster. Whether this is caused by continued input from free‐stream disturbances along the path or to an increased destabilization of the system or both remains unclear. No inflected mean boundary‐layer profiles were registered in careful measurements. Third, there is evidence of roughness‐induced three‐dimensionalization of the wave fronts which leads to earlier secondary subharmonic instabilities and, hence, to turbulence.